Histone acetylation of N-terminal lysines of the core histones has been linked to transcriptional regulation. Our lab has recently identified four distinct native histone acetyltransferase (HAT) complexes from yeast. Two of these enzymes, the 1.8MDa SAGA complex and the 0.8kDa Ada complex both contain the adaptor proteins Ada2, Ada3 and Gcn5. The originally identified coactivator protein Gcn5 is the primary if not only subunit possessing HAT activity within SAGA and Ada. In an attempt to characterize and further investigate the Ada complex, we purified this HAT enzyme over 7 consecutive different chromatographic columns. Highly purified fractions were then analysed by mass spectrometry and peptides were compared to the yeast genome sequence database. Interestingly, several unidentified open reading frames were found to be part of the Ada complex suggesting a new role for these proteins. We are currently generating yeast knock-out strains of these ORFs to further investigate their function within the Ada HAT-complex. Direct investigation whether Ada and SAGA function in transcriptional regulation was done by performing in vitro transcription assays with the HIV-1 promoter. Regulation of this promoter has been linked in vivo to acetylation. The Ada and SAGA complexes stimulate HIV-1 transcription from pre-assembled nucleosomal templates in an acetylCoA-dependent manner. Furthermore, for Ada we find that acetylation of only the histones associated with the HIV-1 template can facilitate transcription.